There is no "typical" ISTP behavior. But if you want to get better grasp of what makes ISTPs tick, spend some time reading through all the ISTP threads that discuss relationships.
Have an ISTP friend I've known for five years, and I've learned a couple very basic, important things about him:
1.) He is of simple wants and desires.
2.) He cannot be trusted. No matter how pure he WANTS his intentions to be, they are never pure. It is ridiculous how deceitful and manipulative he will be to have his cake and eat it too. When all he has to do is just tell me the truth and I'll accept it, like I've accepted and dealt with his tricky, dirty ways of getting what he wants.
3.) When he is uncomfortable talking about something that is happening, he simply pretends it never happened! And hurts people's feelings by doing so. And could really just care less.
4.) He never knows what he wants. I've come to learn that when he wants something, he wants it THEN. Not later, not yesterday, THEN. The only way to deal with him is to take him day by day.
5.) He describes himself as having periods of 'moodiness'. I think these 'moods' come from holding all of his feelings in and neglecting them rather than dealing with them. He pretty much goes from the most calm, collective person I've ever known, to the most vulnerable piece of mush!
Now tell me, is this typical behavior for an ISTP?!